1. Field of the Invention
The present invention relates to a data processing apparatus that has a circuit structure for applying a termination voltage to connecting lines that connect a data processing unit and a main storage device.
2. Description of the Related Art
A double data rate synchronous dynamic random access memory (DDR-SDRAM) has a function that shifts to a mode called a power down mode or a self refresh mode in which power consumption is reduced compared with normal operation. The DDR-SDRAM shifts to the power down mode or the self refresh mode if it is not accessed for more than a predetermined period of time. High speed signal circuits, such as the DDR-SDRAMs, are connected to a termination voltage connected to a data communication line and a controlling line between the data processing unit and a dynamic random access memory (DRAM), via a termination resistor. The termination voltage plays a role in reducing an erroneous operation caused by wave reflection specific to the high speed signal, and a shoulder (stepped waveform) resulting therefrom. However, because the termination resistor behaves as a simple pull-up resistor, when no data processing is performed, it is known that an unnecessary current flows from the termination voltage.
Various technologies that save power consumption in the power down mode or the self refresh mode have been developed. For example, Japanese Patent Application Laid-open No. 2006-331305 discloses a technology to reduce power consumption of the termination voltage and the data processing unit. This is enabled, in the power down mode or the self refresh mode, by providing a termination voltage system corresponding to each terminal logics of the data processing unit, dividing a voltage plane of the substrate, and by selecting whether to interrupt or to continue.
However, the technology disclosed in the Japanese Patent Application Laid-open No. 2006-331305 necessitates providing a power source system for each terminal logics. This structure makes the layout difficult as well as requires more space. A current of several amperes flows through the termination voltage. To cope with a current of this level, a voltage stabilizing unit such as a regulator is used as an interrupting unit. Use of the regulator increases the size and cost of the entire circuit. Moreover, when the regulator is used as the interrupting unit, it takes considerable time to stabilize the voltage when returning to the normal mode from the power down mode or the self refresh mode.